A significant advantage of mass spectrometry is that both qualitative and quantitative data, not available from other analytical techniques, may be reliably obtained. In particular, mass spectrometry (MS) can provide molecular weight, empirical formula, isotope ratios, identification of functional groups, and elucidation of structure.
Historically, MS utilized electron ionization to produce a characteristic fragmentation pattern for an excited ion. Electron ionization (EI) techniques require that the molecule exist as a stable species in the gas phase, and thus the sample must have an appreciable equilibrium vapor pressure in some accessible temperature range. Approximately 80% of the known organic molecules are, however, either nonvolatile, not sufficiently volatile, or not sufficiently stable thermally to allow for the required vaporization. Accordingly, MS techniques were limited to analyzing relatively volatile substances which could be vaporized and ionized in the gas phase without being thermally decomposed.
Recent developments in ionization techniques involve the direct production of gas phase ions from condensed media, and often produce ions characteristic of the intact molecule. These relatively "soft" ionization techniques, such as thermospray, may provide ions characteristic of the intact molecule for many nonvolatile samples. These techniques, when used in conjunction with mass spectrometry, may be used for limited analysis of high molecular weight, nonvolatile, and thermally labile molecules. Thus, although these new ionization techniques have largely have overcome the EI volatile substance limitation, these techniques frequently fail to produce the stable, extensive reproducible fragmentation pattern required for acceptable structural elucidation. Thus, although these new developments have significantly expanded the applications of mass spectrometry, particularly to the life sciences, the poor dissociation efficiency and reproducibility of such high-mass ion fragmentation techniques has still substantially limited the acceptance and utilization of mass spectrometry.
Tandem MS-MS utilizing collisional dissociation techniques provide an acceptable method for yielding useful fragmentation in many cases. However, collision induced dissociation (CID) suffers from several difficiencies, particularly with respect to large, high mass molecules. Since CID causes dissociation by colliding the ions at relatively high kinetic energy with neutral gas molecules, high energy transfer is often accompanied by high momentum transfer, which in turn causes the higher energy portion of the excited ions to be scattered out of the beam. The excitation process which occurs is thus predominately due to internuclear collisions which lead to vibrational and rotational excitation, rather than electronic excitation. While some electronic excitation may occur in grazing collisions, a disproportionate fraction of this excitation energy may be lost, either with the electronic excitation energy of the neutral, or by photon emission. Since the maximum energy available decreases as the mass of the ion increases, CID is substantially ineffective for analyzing large molecules. Finally, fragmentation utilizing the CID process is not as easily reproducible as fragmentation utilizing electron ionization since the energy distribution depends on numerous parameters. Accordingly, tandem MS-MS techniques are also not well suited for analyzing large molecule samples, and have not been widely accepted for studies involving the life sciences. Additional information regarding CID techniques is disclosed in U.S. Pat. Nos. 4,234,791 and 4,536,652.
Lasers with the desired coverage of the UV spectrum for laser photodissociation are generally prohibitively expensive, and are rarely available. Thus, laser photodissociation also has not provided acceptable technology for enabling MS to fully analyze a wide range of samples.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus for use with mass spectrometry are hereinafter described for dissociating ions by electron impact. More particularly, new techniques are hereinafter disclosed for reliably obtaining predictable and reproducible fragmentation patterns from relatively stable, high molecular weight ions, thereby enabling structural information from large, nonvolatile molecules to be reliably obtained.